This invention relates generally to an electrostatographic printing machine, and more particularly concerns an apparatus for developing a latent image.
The process of electrostatographic printing includes both electrographic printing and electrophotographic printing. In both of these processes, a latent image of the original document being reproduced is recorded on a surface. Electrophotographic printing is described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942. As taught therein, a photoconductive member is charged to a substantially uniform potential. The charged photoconductive member is exposed to a light image of the original document. The light image selectively dissipates the charge in the irradiated areas and creates an electrostatic latent image on the photoconductive member. A variation of this process is a chargeless process wherein the photoconductive member is merely exposed to the light image in order to record the latent image thereon. This process does not require charging in order to sensitize the photoconductive surface. Electrographic printing creates a latent image corresponding to the original document to be reproduced without the use of a photoconductive material or a light image.
Irrespective of the method employed in the formation of a latent image, a viewable record thereof is usually produced by depositing particles thereon, i.e., the process of development. Development may be achieved by bringing the latent image into contact with a developer material. Typical developer materials employed in the art generally comprise toner particles, such as heat settable colored thermoplastic particles, which adhere electrostatically to coarser carrier granules, such as ferromagnetic granules. In the alternative, single component developer materials may be employed which utilize magnetic particles.
Various types of developing systems are employed in the art and include, amongst others, cascade development, magnetic brush development, powder cloud development, and liquid development. Magnetic brush systems achieve a substantially uniform density and, therefore, are used in numerous electrostatographic printing machines. Other types of hybrid systems are also frequently employed. For example, a combination of cascade and powder cloud development is also utilized.
Multi-color electrophotographic printing is substantially identical to the heretofore discussed process of black and white printing with the following distinctions. Rather than forming a total light image of the original document, the light image is filtered producing a single color light image which is a partial light image of the original document. This single color light image exposes the charged photoconductive surface to create successive single color latent images thereon. Each single color latent image is developed with particles complementary in color to the color of the filtered light image. These single color powder images are transferred to a sheet of support material, in superimposed registration with one another. In this manner, a multi-colored powder image is formed on the sheet of support material and permanently affixed thereto. This forms a color copy corresponding to the original document.
U.S. Pat. No. 3,563,734 issued to Shelley in 1971 exemplifies chargeless electrophotographic printing and discloses a photoreceptor which is mounted in the form of an advanceable roll and carriage assembly. The photoreceptor is exposed to a light image of an original document and then passes adjacent to a magnetic brush developer assembly. The magnetic brush developer assembly has magnetic, conductive particles adhering thereto. As the photoreceptor, with the charge pattern recorded thereon, passes the magnetic brush assembly, the particles adhering thereto contact the charge pattern and are attracted thereto. These particles are then transferred from the photoreceptor to a sheet of support material secured to a transfer roll. Thereafter, the sheet of support material passes through a fusing device which permanently affixes the powder image thereto forming a black and white copy of the original document. Other relevant patents which exemplify the foregoing type of system are U.S. Pat. Nos. 3,617,124; 3,643,629; 3,739,749; and 3,764,313.
A multi-color variation of this process is disclosed in copending application Ser. No. 518,542 filed in Oct. of 1974, now U.S. Pat. No. 3,963,341. However, it has been found that it is extremely difficult to produce differently colored magnetic particles. Thus, a typical multi-color electrophotographic printing machine requires cyan, magenta and yellow toner particles. This problem may be obviated by employing the non-magnetic toner particles. However, when non-magnetic particles are employed, a magnetic brush system is not a suitable developer unit.
Accordingly, it is the primary object of the present invention to improve electrostatographic printing machines by providing development system suitable for use with single component, non-magnetic particles.